The effective date of the new document should be in 2019. The original effective date of the plan will depend on whether the new document is done as a restatement of the MEP document they previously adopted or as a new plan document.
They were a sponsor of the MEP, so a new plan will be a successor plan under 1.401(k)-2(c)(2)(iii). That means a new plan won't qualify for a short initial plan year for a safe harbor plan 1.401(k)-3(e)(2). Having the new document be a restatement that mirrors the current safe harbor provisions avoids having a short initial plan year in 2019. Being a successor plan also means a new plan won't qualify for the deemed 3% ADP/ACP if it uses prior year testing 1.401(k)-2(c)(2).
Hopefully, the MEP isn't going to offer distributions to active participants since the "new plan" would also be an alternative defined contribution plan under 1.401(k)-1(d)(4)(i).